SiP12109

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Vishay Siliconix

S13-2479-Rev. A, 16-Dec-13

10

Document Number: 62694

THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT

Fig. 21 - CM-COT Operational Principle

The following equation illustrates the relationship between





Once on-time is set, the pseudo constant frequency is then

determined by the following equation:

Loop Stability and Compensator Design

loop stability and transient response purpose. The general concept of this loop design is to introduce a single zero through the

compensator to determine the crossover frequency of overall close loop system.

The overall loop can be broken down into following segments.

The complete loop transfer function is given by:

When:

gm

= Error amplifier transconductance

= Load resistance

= Output capacitor

= Feedback resistor connect to LX

= Feedback resistor connect to ground

= Output impedance of error amplifier = 20 M



= Voltage to current gain = 3

PWM

Fixed ON-time

1

, where K = 17.5 x 10

constant set internally

sw =

=

=

D

1

H

fb

R

fb2

R

------------------------------

=

G

R

1 + sC



1 + sR



------------------------------------------------------------------------- gm

=

H

1

AV

----------------------------------- x

R

1 + sC



1 + sC



--------------------------------------------------------------

=

H

R

fb2

R

------------------------------ x

R

1 + sC



1 + sR



-------------------------------------------------------------------------gm x

1

AV

----------------------------------- x

R

1 + sC



1 + sC



--------------------------------------------------------------

=